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DOC: Update Future (Scala) Chapter. See #1487

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Patrik Nordwall 2011-12-15 18:00:16 +01:00
parent 987a8cc2ee
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akka-docs/scala/code/akka/docs/future/FutureDocSpec.scala Normal file
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@ -0,0 +1,248 @@
package akka.docs.future
import org.scalatest.{ BeforeAndAfterAll, WordSpec }
import org.scalatest.matchers.MustMatchers
import akka.testkit._
import akka.actor.Actor
import akka.actor.Props
import akka.actor.Status.Failure
import akka.dispatch.Future
import akka.dispatch.Await
import akka.util.duration._
object FutureDocSpec {
class MyActor extends Actor {
def receive = {
case x: String ⇒ sender ! x.toUpperCase
case x: Int if x < 0 ⇒ sender ! Failure(new ArithmeticException("Negative values not supported"))
case x: Int ⇒ sender ! x
}
}
case object GetNext
class OddActor extends Actor {
var n = 1
def receive = {
case GetNext ⇒
sender ! n
n += 2
}
}
}
class FutureDocSpec extends AkkaSpec {
import FutureDocSpec._
"demonstrate usage of blocking from actor" in {
val actor = system.actorOf(Props[MyActor])
val msg = "hello"
//#ask-blocking
import akka.dispatch.Await
implicit val timeout = system.settings.ActorTimeout
val future = actor ? msg
val result = Await.result(future, timeout.duration).asInstanceOf[String]
//#ask-blocking
result must be("HELLO")
}
"demonstrate usage of mapTo" in {
val actor = system.actorOf(Props[MyActor])
val msg = "hello"
implicit val timeout = system.settings.ActorTimeout
//#map-to
import akka.dispatch.Future
val future: Future[String] = (actor ? msg).mapTo[String]
//#map-to
Await.result(future, timeout.duration) must be("HELLO")
}
"demonstrate usage of simple future eval" in {
//#future-eval
import akka.dispatch.Await
import akka.dispatch.Future
import akka.util.duration._
implicit def dispatcher = system.dispatcher
val future = Future {
"Hello" + "World"
}
val result = Await.result(future, 1 second)
//#future-eval
result must be("HelloWorld")
}
"demonstrate usage of map" in {
//#map
val f1 = Future {
"Hello" + "World"
}
val f2 = f1 map { x ⇒
x.length
}
val result = Await.result(f2, 1 second)
result must be(10)
f1.value must be(Some(Right("HelloWorld")))
//#map
}
"demonstrate wrong usage of nested map" in {
//#wrong-nested-map
val f1 = Future {
"Hello" + "World"
}
val f2 = Future {
3
}
val f3 = f1 map { x ⇒
f2 map { y ⇒
x.length * y
}
}
//#wrong-nested-map
Await.ready(f3, 1 second)
}
"demonstrate usage of flatMap" in {
//#flat-map
val f1 = Future {
"Hello" + "World"
}
val f2 = Future {
3
}
val f3 = f1 flatMap { x ⇒
f2 map { y ⇒
x.length * y
}
}
val result = Await.result(f3, 1 second)
result must be(30)
//#flat-map
}
"demonstrate usage of for comprehension" in {
//#for-comprehension
val f = for {
a ← Future(10 / 2) // 10 / 2 = 5
b ← Future(a + 1) // 5 + 1 = 6
c ← Future(a - 1) // 5 - 1 = 4
} yield b * c // 6 * 4 = 24
val result = Await.result(f, 1 second)
result must be(24)
//#for-comprehension
}
"demonstrate wrong way of composing" in {
val actor1 = system.actorOf(Props[MyActor])
val actor2 = system.actorOf(Props[MyActor])
val actor3 = system.actorOf(Props[MyActor])
val msg1 = 1
val msg2 = 2
implicit val timeout = system.settings.ActorTimeout
import akka.dispatch.Await
//#composing-wrong
val f1 = actor1 ? msg1
val f2 = actor2 ? msg2
val a = Await.result(f1, 1 second).asInstanceOf[Int]
val b = Await.result(f2, 1 second).asInstanceOf[Int]
val f3 = actor3 ? (a + b)
val result = Await.result(f3, 1 second).asInstanceOf[Int]
//#composing-wrong
result must be(3)
}
"demonstrate composing" in {
val actor1 = system.actorOf(Props[MyActor])
val actor2 = system.actorOf(Props[MyActor])
val actor3 = system.actorOf(Props[MyActor])
val msg1 = 1
val msg2 = 2
implicit val timeout = system.settings.ActorTimeout
import akka.dispatch.Await
//#composing
val f1 = actor1 ? msg1
val f2 = actor2 ? msg2
val f3 = for {
a ← f1.mapTo[Int]
b ← f2.mapTo[Int]
c ← (actor3 ? (a + b)).mapTo[Int]
} yield c
val result = Await.result(f3, 1 second).asInstanceOf[Int]
//#composing
result must be(3)
}
"demonstrate usage of sequence with actors" in {
implicit val timeout = system.settings.ActorTimeout
val oddActor = system.actorOf(Props[OddActor])
//#sequence-ask
// oddActor returns odd numbers sequentially from 1 as a List[Future[Int]]
val listOfFutures = List.fill(100)((oddActor ? GetNext).mapTo[Int])
// now we have a Future[List[Int]]
val futureList = Future.sequence(listOfFutures)
// Find the sum of the odd numbers
val oddSum = Await.result(futureList.map(_.sum), 1 second).asInstanceOf[Int]
oddSum must be(10000)
//#sequence-ask
}
"demonstrate usage of sequence" in {
//#sequence
val futureList = Future.sequence((1 to 100).toList.map(x ⇒ Future(x * 2 - 1)))
val oddSum = Await.result(futureList.map(_.sum), 1 second).asInstanceOf[Int]
oddSum must be(10000)
//#sequence
}
"demonstrate usage of traverse" in {
//#traverse
val futureList = Future.traverse((1 to 100).toList)(x ⇒ Future(x * 2 - 1))
val oddSum = Await.result(futureList.map(_.sum), 1 second).asInstanceOf[Int]
oddSum must be(10000)
//#traverse
}
"demonstrate usage of fold" in {
//#fold
val futures = for (i ← 1 to 1000) yield Future(i * 2) // Create a sequence of Futures
val futureSum = Future.fold(futures)(0)(_ + _)
Await.result(futureSum, 1 second) must be(1001000)
//#fold
}
"demonstrate usage of reduce" in {
//#reduce
val futures = for (i ← 1 to 1000) yield Future(i * 2) // Create a sequence of Futures
val futureSum = Future.reduce(futures)(_ + _)
Await.result(futureSum, 1 second) must be(1001000)
//#reduce
}
"demonstrate usage of recover" in {
implicit val timeout = system.settings.ActorTimeout
val actor = system.actorOf(Props[MyActor])
val msg1 = -1
//#recover
val future = actor ? msg1 recover {
case e: ArithmeticException ⇒ 0
}
//#recover
Await.result(future, 1 second) must be(0)
}
}

222
akka-docs/scala/futures.rst
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@ -11,6 +11,7 @@ Introduction
------------ ------------
In Akka, a `Future <http://en.wikipedia.org/wiki/Futures_and_promises>`_ is a data structure used to retrieve the result of some concurrent operation. This operation is usually performed by an ``Actor`` or by the ``Dispatcher`` directly. This result can be accessed synchronously (blocking) or asynchronously (non-blocking). In Akka, a `Future <http://en.wikipedia.org/wiki/Futures_and_promises>`_ is a data structure used to retrieve the result of some concurrent operation. This operation is usually performed by an ``Actor`` or by the ``Dispatcher`` directly. This result can be accessed synchronously (blocking) or asynchronously (non-blocking).
Asynchronous usage is strongly recommended.
Use with Actors Use with Actors
--------------- ---------------
@ -19,202 +20,113 @@ There are generally two ways of getting a reply from an ``Actor``: the first is
Using an ``Actor``\'s ``?`` method to send a message will return a Future. To wait for and retrieve the actual result the simplest method is: Using an ``Actor``\'s ``?`` method to send a message will return a Future. To wait for and retrieve the actual result the simplest method is:
.. code-block:: scala .. includecode:: code/akka/docs/future/FutureDocSpec.scala
:include: ask-blocking
val future = actor ? msg This will cause the current thread to block and wait for the ``Actor`` to 'complete' the ``Future`` with it's reply. Blocking is discouraged though as it can cause performance problem. Alternatives to blocking are discussed further within this documentation.
val result = future.get() Also note that the ``Future`` returned by an ``Actor`` is a ``Future[Any]`` since an ``Actor`` is dynamic. That is why the ``asInstanceOf`` is used in the above sample.
When using non-blocking it is better to use the ``mapTo`` method to safely try to cast a ``Future`` to an expected type:
This will cause the current thread to block and wait for the ``Actor`` to 'complete' the ``Future`` with it's reply. Blocking is discouraged though as it can cause performance problem. Alternatives to blocking are discussed futher within this documentation. Also note that the ``Future`` returned by an ``Actor`` is a ``Future[Any]`` since an ``Actor`` is dynamic. To safely try to cast a ``Future`` to an expected type the ``mapTo`` method may be used: .. includecode:: code/akka/docs/future/FutureDocSpec.scala
:include: map-to
.. code-block:: scala The ``mapTo`` method will return a new ``Future`` that contains the result if the cast was successful, or a ``ClassCastException`` if not. Handling ``Exception``\s will be discussed further within this documentation.
val future = actor ? msg
val result = future.mapTo[String].get()
The ``mapTo`` method will return a new ``Future`` that contains the result if the cast was successful, or a ``ClassCastException`` if not. Handling ``Exception``\s will be disccused further within this documentation.
Use Directly Use Directly
------------ ------------
A common use case within Akka is to have some computation performed concurrently without needing the extra utility of an ``Actor``. If you find yourself creating a pool of ``Actor``\s for the sole reason of performing a calculation in parallel, there is an easier (and faster) way: A common use case within Akka is to have some computation performed concurrently without needing the extra utility of an ``Actor``. If you find yourself creating a pool of ``Actor``\s for the sole reason of performing a calculation in parallel, there is an easier (and faster) way:
.. code-block:: scala .. includecode:: code/akka/docs/future/FutureDocSpec.scala
:include: future-eval
import akka.dispatch.Future
val future = Future {
"Hello" + "World"
}
val result = future.get()
In the above code the block passed to ``Future`` will be executed by the default ``Dispatcher``, with the return value of the block used to complete the ``Future`` (in this case, the result would be the string: "HelloWorld"). Unlike a ``Future`` that is returned from an ``Actor``, this ``Future`` is properly typed, and we also avoid the overhead of managing an ``Actor``. In the above code the block passed to ``Future`` will be executed by the default ``Dispatcher``, with the return value of the block used to complete the ``Future`` (in this case, the result would be the string: "HelloWorld"). Unlike a ``Future`` that is returned from an ``Actor``, this ``Future`` is properly typed, and we also avoid the overhead of managing an ``Actor``.
Functional Futures Functional Futures
------------------ ------------------
A recent addition to Akka's ``Future`` is several monadic methods that are very similar to the ones used by Scala's collections. These allow you to create 'pipelines' or 'streams' that the result will travel through. Akka's ``Future`` has several monadic methods that are very similar to the ones used by Scala's collections. These allow you to create 'pipelines' or 'streams' that the result will travel through.
Future is a Monad Future is a Monad
^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^^
The first method for working with ``Future`` functionally is ``map``. This method takes a ``Function`` which performs some operation on the result of the ``Future``, and returning a new result. The return value of the ``map`` method is another ``Future`` that will contain the new result: The first method for working with ``Future`` functionally is ``map``. This method takes a ``Function`` which performs some operation on the result of the ``Future``, and returning a new result. The return value of the ``map`` method is another ``Future`` that will contain the new result:
.. code-block:: scala .. includecode:: code/akka/docs/future/FutureDocSpec.scala
:include: map
val f1 = Future { In this example we are joining two strings together within a Future. Instead of waiting for this to complete, we apply our function that calculates the length of the string using the ``map`` method. Now we have a second ``Future`` that will eventually contain an ``Int``. When our original ``Future`` completes, it will also apply our function and complete the second ``Future`` with it's result. When we finally get the result, it will contain the number 10. Our original ``Future`` still contains the string "HelloWorld" and is unaffected by the ``map``.
"Hello" + "World"
}
val f2 = f1 map { x =>
x.length
}
val result = f2.get()
In this example we are joining two strings together within a Future. Instead of waiting for this to complete, we apply our function that calculates the length of the string using the ``map`` method. Now we have a second ``Future`` that will eventually contain an ``Int``. When our original ``Future`` completes, it will also apply our function and complete the second ``Future`` with it's result. When we finally ``get`` the result, it will contain the number 10. Our original ``Future`` still contains the string "HelloWorld" and is unaffected by the ``map``.
The ``map`` method is fine if we are modifying a single ``Future``, but if 2 or more ``Future``\s are involved ``map`` will not allow you to combine them together: The ``map`` method is fine if we are modifying a single ``Future``, but if 2 or more ``Future``\s are involved ``map`` will not allow you to combine them together:
.. code-block:: scala .. includecode:: code/akka/docs/future/FutureDocSpec.scala
:include: wrong-nested-map
val f1 = Future { ``f3`` is a ``Future[Future[Int]]`` instead of the desired ``Future[Int]``. Instead, the ``flatMap`` method should be used:
"Hello" + "World"
}
val f2 = Future { .. includecode:: code/akka/docs/future/FutureDocSpec.scala
3 :include: flat-map
}
val f3 = f1 map { x =>
f2 map { y =>
x.length * y
}
}
val result = f3.get().get()
The ``get`` method had to be used twice because ``f3`` is a ``Future[Future[Int]]`` instead of the desired ``Future[Int]``. Instead, the ``flatMap`` method should be used:
.. code-block:: scala
val f1 = Future {
"Hello" + "World"
}
val f2 = Future {
3
}
val f3 = f1 flatMap { x =>
f2 map { y =>
x.length * y
}
}
val result = f3.get()
For Comprehensions For Comprehensions
^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^^^
Since ``Future`` has a ``map`` and ``flatMap`` method it can be easily used in a 'for comprehension': Since ``Future`` has a ``map`` and ``flatMap`` method it can be easily used in a 'for comprehension':
.. code-block:: scala .. includecode:: code/akka/docs/future/FutureDocSpec.scala
:include: for-comprehension
val f = for {
a <- Future(10 / 2) // 10 / 2 = 5
b <- Future(a + 1) // 5 + 1 = 6
c <- Future(a - 1) // 5 - 1 = 4
} yield b * c // 6 * 4 = 24
val result = f.get()
Something to keep in mind when doing this is even though it looks like parts of the above example can run in parallel, each step of the for comprehension is run sequentially. This will happen on separate threads for each step but there isn't much benefit over running the calculations all within a single ``Future``. The real benefit comes when the ``Future``\s are created first, and then combining them together. Something to keep in mind when doing this is even though it looks like parts of the above example can run in parallel, each step of the for comprehension is run sequentially. This will happen on separate threads for each step but there isn't much benefit over running the calculations all within a single ``Future``. The real benefit comes when the ``Future``\s are created first, and then combining them together.
Composing Futures Composing Futures
^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^^
The example for comprehension above is an example of composing ``Future``\s. A common use case for this is combining the replies of several ``Actor``\s into a single calculation without resorting to calling ``get`` or ``await`` to block for each result. First an example of using ``get``: The example for comprehension above is an example of composing ``Future``\s. A common use case for this is combining the replies of several ``Actor``\s into a single calculation without resorting to calling ``Await.result`` or ``Await.ready`` to block for each result. First an example of using ``Await.result``:
.. code-block:: scala .. includecode:: code/akka/docs/future/FutureDocSpec.scala
:include: composing-wrong
val f1 = actor1 ? msg1 Here we wait for the results from the first 2 ``Actor``\s before sending that result to the third ``Actor``. We called ``Await.result`` 3 times, which caused our little program to block 3 times before getting our final result. Now compare that to this example:
val f2 = actor2 ? msg2
val a = f1.mapTo[Int].get() .. includecode:: code/akka/docs/future/FutureDocSpec.scala
val b = f2.mapTo[Int].get() :include: composing
val f3 = actor3 ? (a + b)
val result = f3.mapTo[String].get()
Here we wait for the results from the first 2 ``Actor``\s before sending that result to the third ``Actor``. We called ``get`` 3 times, which caused our little program to block 3 times before getting our final result. Now compare that to this example:
.. code-block:: scala
val f1 = actor1 ? msg1
val f2 = actor2 ? msg2
val f3 = for {
a <- f1.mapTo[Int]
b <- f2.mapTo[Int]
c <- (actor3 ? (a + b)).mapTo[String]
} yield c
val result = f3.get()
Here we have 2 actors processing a single message each. Once the 2 results are available (note that we don't block to get these results!), they are being added together and sent to a third ``Actor``, which replies with a string, which we assign to 'result'. Here we have 2 actors processing a single message each. Once the 2 results are available (note that we don't block to get these results!), they are being added together and sent to a third ``Actor``, which replies with a string, which we assign to 'result'.
This is fine when dealing with a known amount of Actors, but can grow unwieldy if we have more then a handful. The ``sequence`` and ``traverse`` helper methods can make it easier to handle more complex use cases. Both of these methods are ways of turning, for a subclass ``T`` of ``Traversable``, ``T[Future[A]]`` into a ``Future[T[A]]``. For example: This is fine when dealing with a known amount of Actors, but can grow unwieldy if we have more then a handful. The ``sequence`` and ``traverse`` helper methods can make it easier to handle more complex use cases. Both of these methods are ways of turning, for a subclass ``T`` of ``Traversable``, ``T[Future[A]]`` into a ``Future[T[A]]``. For example:
.. code-block:: scala .. includecode:: code/akka/docs/future/FutureDocSpec.scala
:include: sequence-ask
// oddActor returns odd numbers sequentially from 1 as a List[Future[Int]]
val listOfFutures = List.fill(100)((oddActor ? GetNext).mapTo[Int])
// now we have a Future[List[Int]]
val futureList = Future.sequence(listOfFutures)
// Find the sum of the odd numbers
val oddSum = futureList.map(_.sum).get()
To better explain what happened in the example, ``Future.sequence`` is taking the ``List[Future[Int]]`` and turning it into a ``Future[List[Int]]``. We can then use ``map`` to work with the ``List[Int]`` directly, and we find the sum of the ``List``. To better explain what happened in the example, ``Future.sequence`` is taking the ``List[Future[Int]]`` and turning it into a ``Future[List[Int]]``. We can then use ``map`` to work with the ``List[Int]`` directly, and we find the sum of the ``List``.
The ``traverse`` method is similar to ``sequence``, but it takes a ``T[A]`` and a function ``A => Future[B]`` to return a ``Future[T[B]]``, where ``T`` is again a subclass of Traversable. For example, to use ``traverse`` to sum the first 100 odd numbers: The ``traverse`` method is similar to ``sequence``, but it takes a ``T[A]`` and a function ``A => Future[B]`` to return a ``Future[T[B]]``, where ``T`` is again a subclass of Traversable. For example, to use ``traverse`` to sum the first 100 odd numbers:
.. code-block:: scala .. includecode:: code/akka/docs/future/FutureDocSpec.scala
:include: traverse
val oddSum = Future.traverse((1 to 100).toList)(x => Future(x * 2 - 1)).map(_.sum).get()
This is the same result as this example: This is the same result as this example:
.. code-block:: scala .. includecode:: code/akka/docs/future/FutureDocSpec.scala
:include: sequence
val oddSum = Future.sequence((1 to 100).toList.map(x => Future(x * 2 - 1))).map(_.sum).get()
But it may be faster to use ``traverse`` as it doesn't have to create an intermediate ``List[Future[Int]]``. But it may be faster to use ``traverse`` as it doesn't have to create an intermediate ``List[Future[Int]]``.
Then there's a method that's called ``fold`` that takes a start-value, a sequence of ``Future``\s and a function from the type of the start-value and the type of the futures and returns something with the same type as the start-value, and then applies the function to all elements in the sequence of futures, non-blockingly, the execution will run on the Thread of the last completing Future in the sequence. Then there's a method that's called ``fold`` that takes a start-value, a sequence of ``Future``\s and a function from the type of the start-value and the type of the futures and returns something with the same type as the start-value, and then applies the function to all elements in the sequence of futures, non-blockingly, the execution will run on the Thread of the last completing Future in the sequence.
.. code-block:: scala .. includecode:: code/akka/docs/future/FutureDocSpec.scala
:include: fold
val futures = for(i <- 1 to 1000) yield Future(i * 2) // Create a sequence of Futures
val futureSum = Future.fold(0)(futures)(_ + _)
That's all it takes! That's all it takes!
If the sequence passed to ``fold`` is empty, it will return the start-value, in the case above, that will be 0. In some cases you don't have a start-value and you're able to use the value of the first completing Future in the sequence as the start-value, you can use ``reduce``, it works like this: If the sequence passed to ``fold`` is empty, it will return the start-value, in the case above, that will be 0. In some cases you don't have a start-value and you're able to use the value of the first completing Future in the sequence as the start-value, you can use ``reduce``, it works like this:
.. code-block:: scala .. includecode:: code/akka/docs/future/FutureDocSpec.scala
:include: reduce
val futures = for(i <- 1 to 1000) yield Future(i * 2) // Create a sequence of Futures Same as with ``fold``, the execution will be done by the Thread that completes the last of the Futures, you can also parallelize it by chunking your futures into sub-sequences and reduce them, and then reduce the reduced results again.
val futureSum = Future.reduce(futures)(_ + _) This is just a sample of what can be done, but to use more advanced techniques it is easier to take advantage of Scalaz.
Same as with ``fold``, the execution will be done by the Thread that completes the last of the Futures, you can also parallize it by chunking your futures into sub-sequences and reduce them, and then reduce the reduced results again.
This is just a sample of what can be done, but to use more advanced techniques it is easier to take advantage of Scalaz, which Akka has support for in its akka-scalaz module.
Scalaz Scalaz
@ -229,58 +141,12 @@ complete support of programming in a functional style.
Exceptions Exceptions
---------- ----------
Since the result of a ``Future`` is created concurrently to the rest of the program, exceptions must be handled differently. It doesn't matter if an ``Actor`` or the dispatcher is completing the ``Future``, if an ``Exception`` is caught the ``Future`` will contain it instead of a valid result. If a ``Future`` does contain an ``Exception``, calling ``get`` will cause it to be thrown again so it can be handled properly. Since the result of a ``Future`` is created concurrently to the rest of the program, exceptions must be handled differently. It doesn't matter if an ``Actor`` or the dispatcher is completing the ``Future``, if an ``Exception`` is caught the ``Future`` will contain it instead of a valid result. If a ``Future`` does contain an ``Exception``, calling ``Await.result`` will cause it to be thrown again so it can be handled properly.
It is also possible to handle an ``Exception`` by returning a different result. This is done with the ``recover`` method. For example: It is also possible to handle an ``Exception`` by returning a different result. This is done with the ``recover`` method. For example:
.. code-block:: scala .. includecode:: code/akka/docs/future/FutureDocSpec.scala
:include: recover
val future = actor ? msg1 recover { In this example, if the actor replied with a ``akka.actor.Status.Failure`` containing the ``ArithmeticException``, our ``Future`` would have a result of 0. The ``recover`` method works very similarly to the standard try/catch blocks, so multiple ``Exception``\s can be handled in this manner, and if an ``Exception`` is not handled this way it will be behave as if we hadn't used the ``recover`` method.
case e: ArithmeticException => 0
}
In this example, if an ``ArithmeticException`` was thrown while the ``Actor`` processed the message, our ``Future`` would have a result of 0. The ``recover`` method works very similarly to the standard try/catch blocks, so multiple ``Exception``\s can be handled in this manner, and if an ``Exception`` is not handled this way it will be behave as if we hadn't used the ``recover`` method.
Timeouts
--------
Waiting forever for a ``Future`` to be completed can be dangerous. It could cause your program to block indefinitly or produce a memory leak. ``Future`` has support for a timeout already builtin with a default of 5 seconds (taken from :ref:`configuration`). A timeout is an instance of ``akka.util.Timeout`` which contains an ``akka.util.Duration``. A ``Duration`` can be finite, which needs a length and unit type, or infinite. An infinite ``Timeout`` can be dangerous since it will never actually expire.
A different ``Timeout`` can be supplied either explicitly or implicitly when a ``Future`` is created. An implicit ``Timeout`` has the benefit of being usable by a for-comprehension as well as being picked up by any methods looking for an implicit ``Timeout``, while an explicit ``Timeout`` can be used in a more controlled manner.
Explicit ``Timeout`` example:
.. code-block:: scala
import akka.util.duration._
val future1 = Future( { runSomething }, 1 second)
val future2 = future1.map(doSomethingElse)(1500 millis)
Implicit ``Timeout`` example:
.. code-block:: scala
import akka.util.Timeout
import akka.util.duration._
implicit val longTimeout = Timeout(1 minute)
val future1 = Future { runSomething }
val future2 = future1 map doSomethingElse
An important note: when explicitly providing a ``Timeout`` it is fine to just use a ``Duration`` (like in the above explicit ``Timeout`` example). An implicit ``Duration`` will be ignored if an implicit ``Timeout`` is required. Due to this, in the above implicit example the ``Duration`` is wrapped within a ``Timeout``.
If the timeout is reached the ``Future`` becomes unusable, even if an attempt is made to complete it. It is possible to have a ``Future`` handle a timeout, if needed, with the ``onTimeout`` and ``orElse`` methods:
.. code-block:: scala
val future1 = actor ? msg onTimeout { _ =>
println("Timed out!")
}
val future2 = actor ? msg orElse "Timed out!"
Using ``onTimeout`` will cause the supplied block to be executed if the ``Future`` expires, while ``orElse`` will complete the ``Future`` with the supplied value if the ``Future`` expires.